Effects of a reduced calcium, phosphorus and protein intake and of benzoic acid on calcium and phosphorus metabolism of growing pigs

In order to minimise environmental pollution, many pig feeds contain low phosphorus and protein concentrations as well as benzoic acid (BA), an additive which reduces ammonia formation in the slurry. Since both a low P intake and metabolic acidosis compromise bone mineralisation, the effect of a diet with a low concentration of calcium (Ca), phosphorus (P) and crude protein (CP) and the effect of BA on Ca and P metabolism were examined in a 2 × 2 two-factorial experiment using pigs from 13 to 64 kg body weight (BW). Compared to the control piglet and grower diets (8.7 and 6.9 g Ca; 6.9 and 5.3 g P; 172 and 156 g CP per kg, respectively), the intake of the low nutrient piglet and grower diets (5.3 g Ca; 4.3 and 4.0 g P; 154 and 147 g CP per kg, respectively, both supplemented with 1500 U/kg microbial phytase) reduced (P<0.01) Ca and P retention by 27% and 24%, respectively, reduced (P<0.05) the growth rate of the piglets by 7%, and decreased (P<0.05) the bone breaking strength and bone mineral content (P<0.01) by 5% in the animals which were slaughtered at 64 kg BW. Benzoic acid (5 and 10 g per kg piglet and grower diet, respectively) did not influence (P>0.05) the apparent digestibility of Ca, increased the apparent digestibility of P (P<0.05) by 5% and increased the urinary Ca and P output (P<0.01) by 70% and 83%, respectively, but had no effect (P>0.05) on the proportion of ingested Ca and P which was retained. Furthermore, BA increased (P<0.01) the serum activity of the bone formation marker alkaline phosphatase at 25 and 40 kg BW by 17% and 13%, respectively and decreased (P<0.01) the concentration of the bone resorption marker serum crosslaps at 25 kg BW by 12%, implying that BA affected bone metabolism at 25 and 40 kg BW. Since BA neither affected the blood variables at 60 kg nor the bone breaking strength and bone mineral content, any possible negative effect of BA on bone metabolism of the piglets and of the young growing animals thus seems to have disappeared during the last period of the grower period. In conclusion, the slight metabolic acidosis caused by BA had no lasting negative effects on the bones of the growing pigs.